This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-39001 filed on Feb. 15, 2001.
1. Field of the Invention
The present invention relates to a capacitive moisture sensor and a fabrication method for the sensor. The capacitive moisture sensor includes a pair of electrodes facing each other with moisture-sensitive material interposed therebetween, whose dielectric constant varies in response to ambient moisture. Ambient moisture is sensed on a basis of the capacitance between the electrodes, which is correlated with the dielectric constant.
2. Background of the Invention
A capacitive moisture sensor is utilized to measure indoor moisture for an air conditioner, outdoor moisture for meteorological observation, and so on. A capacitive moisture sensor that is highly sensitive, compact, reliable, and produced at low costs is demanded.
In order to increase the capacitance sensitivity between the electrodes, it is necessary to increase the size of the surfaces of the electrodes, which face each other. In proposed capacitive moisture sensors such as in JP-U-5-23124 and JP-A-64-86053, the two electrodes are formed from a metal film deposited on the surface of an insulator. Therefore, if it is possible to use only a vertical side wall of a thick metal film as the facing surfaces of the electrodes, the area size is significantly increased with simple configuration and compact size. However, in the proposed sensors, the two electrodes are formed from a thin metal film, so that the thickness of the thin film is restricted by film forming method, etching method, increased film internal stress, film brittleness due to the stress, or the like. Therefore, to provide sufficient facing area, it is necessary to extend horizontally the electrodes or use the horizontal surface of the electrodes. As a result, the sensor becomes bulky, or the fabrication process becomes complicated.
In addition, in the proposed capacitive moisture sensors, the electrodes are made of metal, so that the electrodes are vulnerable to corrosion caused by moisture.
The present invention has been made in view of the above aspects with an object to provide a capacitive moisture sensor that is preferably sensitive, compact, reliable, and produced at low costs.
In the present invention, a capacitive moisture sensor is made from an SOI, i.e., silicon-on-insulator, substrate, and electrodes for moisture sensing are formed from a thick silicon layer. For example, the thickness may be as thick as 10-15 xcexcm and may be thicker. Thereby, the area of the electrodes facing each other is drastically enlarged in comparison with those of the proposed sensors. That is, the sensitivity to moisture in the capacitance between the electrodes is readily increased without horizontally widening the electrodes, namely, without enlarging sensor size. In addition, the electrodes are made of silicon, so that corrosion resistivity against moisture is significantly improved in comparison with the proposed metal electrodes.